1. Field of the Invention
The present invention relates to electrical load protection devices in general and particularly to a device for monitoring line conditions in a three phase power supply line.
2. Background of the Prior Art
Existing power monitoring circuits for monitoring line conductors in a three phase power line typically comprise parallel-connected DC power supplies for powering various circuit components (e.g. indicator components, sensing circuitry components, and relays). This arrangement results in excessive power consumption levels requiring that these circuits be made using high voltage and high cost circuit components. In addition, existing power monitoring circuits must be housed in large, expensive protective housings account for the heating effects attendant to the high power consumption levels.
Efforts in the past to reduce power consumption levels in three phase monitoring circuits have often resulted in further increases in circuit component costs. For example, in U.S. Pat. No. 5,548,207 a power monitoring circuit is described in which the line voltage of the line being monitored is reduced using a three phase step-down isolation transformer. While the circuit features reduced power consumption, it is estimated that the step-down isolation transformer shown in the ""207 patent accounts for at least 50 percent of the total costs of the ""207 circuit.
There is a need therefore for a three phase power monitor which consumes reduced power, comprises low cost circuit components, is readily packaged in a small, inexpensive housing, and which, at the same time, accurately detects line voltage characteristics.
According to its major aspects and broadly stated, the invention is a low cost, low power consumption power monitoring circuit for monitoring line conditions in a three phase power supply mains.
The power monitoring circuit includes a signal analyzer and plurality of series-connected power supplies for powering various circuit components, including such components as a signal analyzer and a relay coil. The series connection of the power supplies reduces the power consumption of the circuit, thereby reducing the required voltage ratings and cost of the circuit components as well as reducing the required heat stress rating and cost of the circuit""s packaging. According to the invention, the signal input into the signal analyzer for analyzing line conditions is a signal indicative of the current supplying the series-connected power supplies. This current-indicating signal may be developed with use of a current sensor comprising a sensing resistor and voltage divider combination, or another type of current sensor such as a linear optocoupler, a DC current sensor, or a Hall-effect sensor.
A power monitoring circuit according to the invention preferably further includes a plurality of voltage reducing resistors arranged between the power supply mains and the bridge rectifier of the power monitoring circuit. While these voltage reducing resistors operate to reduce voltage applied to some of the circuit components and therefore further reduce component cost, they tend to introduce a distortion component into the input signal input into the circuit""s signal analyzer.
In another aspect of the invention, the circuit signal analyzer includes a programmed microprocessor which has been previously programmed to accurately sense line conditions based on the input signal in spite of the presence of a distortion component in the input signal. Specifically, the signal analyzer may include a program having a mathematical formula relating the distorted input signal to line conditions or else may include a programmed lookup table correlating input signals with associated line conditions. The distortion backout program may be developed prior to the circuit being placed into operation by analyzing the current-indicating input signal under various experimentally controlled line conditions.
These and other details, advantages and benefits of the present invention will become apparent from the detailed description of the preferred embodiment hereinbelow.